1. Field of the Invention
The present disclosure relates generally to medium access control with power management methods and systems, and, more particularly to methods and systems for use in an ad hoc wireless network, where stations can transmit data accurately when operating in the power saving mode.
2. Description of the Related Art
Currently, the IEEE 802.11 is the most popular international medium access control (MAC) standard for WLANs (Wireless Local Area Networks). Based on the network architecture, wireless networks can be approximately divided two classes: infrastructure WLANs and ad hoc networks. FIG. 1 is a schematic diagram illustrating an ad hoc network. As shown in FIG. 1, each station (110, 120, 130, 140 and 150) can dynamically communicate with adjacent stations for data transmissions.
FIG. 2 is a schematic diagram illustrating a power consumption model for a general wireless network interface card (or adapter). Each station can stay in one of the transmission, reception, listen, or doze states. As shown in FIG. 2, the power consumption is approximately between 1.6 W to 1.2 W when the station stays in either the transmission, reception, or listen states, but is close to zero when staying in the doze state. In IEEE 802.11 power management for ad hoc networks, time is divided into fixed-sized BIs (Beacon Intervals), each of which contains an ATIM (Announcement Traffic Indication Message) window. Each station in a power saving mode (or called “power-saving station) must wake up at the beginning of each BI and remain awake in the ATIM window, awaiting the ATIM frame from other stations. If no ATIM frame is received in the ATIM window, then that station may enter the doze state after the ATIM window ends. If an ATIM frame is received in the ATIM window, then the station should reply the ATIM ACK (Acknowledgement) to the station transmitting the ATIM frame, and remains awake after the ATIM window ends. After the end of the ATIM window, the station which sending ATIM frames should use the DCF (distributed coordination function) procedure to transmit the buffered data frames to its intended destination, and the destination should acknowledge its receipt. For a more detailed presentation, please refer to IEEE 802.11 specification.
FIG. 3 is a schematic diagram illustrating an example of power management in an ad hoc network based on IEEE 802.11. As shown in FIG. 3, when a BI 1 begins (the timing is referred to as TBTT (Target Beacon Transmission Time)), stations X and Y compete to transmit a beacon frame for timing synchronization. It is understood that, in the example of FIG. 3, station X transmits a beacon frame for timing synchronization between stations comprising station X in the network. Since no ATIM frame is received in the ATIM window (AW for short), both stations X and Y enter the doze state (S) after the AW ends. BI 2 begins, and station X successfully transmits a beacon frame. Since station X receives an ATIM frame A from station Y in the AW of BI 2, station X returns a ATIM ACK a to station Y, and remains awake after the AW ends. After the AW ends, station Y can transmit a data frame D to station X, and station X returns a data ACK d to station Y after receiving the data frame D.
As described, in IEEE 802.11, each station in power saving mode must wake up in the ATIM window of “every” BI even if its battery power is low or there is no traffic for it. Hence we hope that each a power saving station can dynamically tune its listen interval (the number of BIs between two adjoining AWs) according to the remaining battery power status or other QoS considerations. Obviously, the value of LI is fixed at “one” in IEEE 802.11. In the invention, the LI of a power saving station can be adjusted according to parameters of quality of service or the remaining power of the station, substantially reducing power consumption on station.